Many different types of DC to AC inverters have been designed and are utilized in practice. One such inverter, disclosed in U.S. Pat. No. 3,832,643, to Van Heyningen et al, employs a tapped DC power supply, with each tap being connected via a separate branch circuit to a load. Each branch circuit includes a transistor switch activated so that at any particular time only one switch couples current from one of the taps to the load. By sequentially energising the different switches the voltage impressed on the load is varied. The several branches are driven by a common signal source, having a wave shape that is reflected in the wave shape of voltage and/or current applied to a load.
In one embodiment, different switches are activated at different times in response to the signal source voltage by connecting Zener diodes having different thresholds to base control electrodes of the transistors. Each Zener diode is connected in series with a transformer winding coupled to the signal source, with the series combination being connected between the base and emitter of a separate switching transistor. In response to the signal source being in a predetermined range, only one of the transistors is activated into a conducting state. The transistor of each branch conducts between cut off and saturation.
The power dissipated by each amplifying transistor in each branch is minimized by selecting the voltage, E, applied to each branch such that at one point during one half of the quarter cycles of the control source the dissipated power in the transistor is zero. In particular, the power dissipated in the transistor of each branch is minimized by varying the applied voltage in steps during each cycle of the input signal to minimize the expression E-IR, where E is the voltage applied across the branch, I is the current flowing in the branch and R is the resistance of each branch, while conducting.
In a first embodiment, a different voltage E is applied to each of the different branches and the resistance R of the different branches is the same. Because of the different values of E, different currents flow through the different branches. Thus, if a control input signal has a small amplitude, a first branch conducts and applies a low voltage to the load. When the control signal has a higher value a second branch conducts and applies a second, higher voltage to the load. The first and second branches insert the same resistance between the voltage they apply to the load. The values of E and R are selected so that E-IR is minimized in each branch at one instant while it is conducting.
In a second embodiment, the same voltage is applied to each branch, but the resistance and current of each branch differ while the branch is conducting. When the signal source has a small amplitude, a relatively small current flows through a first conducting branch to the load, because the branch has a relatively high impedance. In contrast, when the control input signal has a high amplitude, a large current is supplied by a second conducting branch to the load, due to the second conducting branch having a relatively low resistance. The values of I and R are chosen such that the product IR is substantially equal to the common voltage E applied to all of the branches, so that the expression E-IR is substantially zero for each branch.
A possible problem with the prior art device disclosed in the Van Heyningen et al patent is that it is dependent upon the emitter collector impedance of a number of power transistors being precisely equal to each other or precisely different from each other by predetermined amounts. If the conducting emitter collector impedance of the transistor switches in the different branch circuits in the prior art circuit are not precisely determined, it is not possible to minimize E-IR. Another possible problem with the Van Heyningen et al circuit is obtaining Zener diodes having sufficiently precise voltage characteristics to enable the different branches to be activated into the conducting and cut off states in response to different voltages of the control signal source.
It is, accordingly, an object of the present invention to provide a new and improved high efficiency DC to AC inverter employing plural switched branches, and method of operating same.
Another object of the present invention is to provide a new and improved high efficiency DC to AC inverter having switched branches employing virtually identical transistor switching circuits.
Still another object of the present invention is to provide a new and improved high efficiency DC to AC inverter having switched branch circuits connected in a relatively simple circuit configuration, wherein the switching for each branch is determined by the amplitude of a conventional gradually varying signal source.